Unless you have not reported any Forms 1099-R for distributions from employer plans, the behavior that you are observing is likely the result of the limit for the Maryland pension exclusion being reduced by the amount of the individual's Social Security income, resulting in an increase in the amount of taxable pension distributions. It's not that the Social Security payments themselves are being taxed. The Maryland pension exclusion appears on Form 502 line 10a.
Check to see if you entered your SSA1099 in the right place on your federal form.
Go to Federal> Wages & Income>>Retirement Plans and Social Security (SSA1099 and 1099RRB) to enter your SSA1099.
Unless you have not reported any Forms 1099-R for distributions from employer plans, the behavior that you are observing is likely the result of the limit for the Maryland pension exclusion being reduced by the amount of the individual's Social Security income, resulting in an increase in the amount of taxable pension distributions. It's not that the Social Security payments themselves are being taxed. The Maryland pension exclusion appears on Form 502 line 10a.